Metal nanowire ink for the formation of transparent conductive films with fused networks

ABSTRACT

Fusing nanowire inks are described that can also comprise a hydrophilic polymer binder, such as a cellulose based binder. The fusing nanowire inks can be deposited onto a substrate surface and dried to drive the fusing process. Transparent conductive films can be formed with desirable properties.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of U.S. patent application Ser. No.16/952,372 filed Nov. 19, 2020 to Li et al., entitled “Metal NanowireInk and Method for Forming Conductive Film,” now U.S. Pat. No.11,512,215, which is a continuation of U.S. patent application Ser. No.16/127,462 filed Sep. 11, 2018 to Li et al., now U.S. Pat. No.10,870,772 B2, entitled “Transparent Conductive Films with FusedNetworks,” which is a continuation of U.S. patent application Ser. No.15/247,533 filed on Aug. 25, 2016 to Li et al., now U.S. Pat. No.10,100,213, entitled “Metal Nanowire Inks for the Formation ofTransparent Conductive Films with Fused Networks,” which is acontinuation of U.S. patent application Ser. No. 14/848,697 filed onSep. 9, 2015 to Li et al., now U.S. Pat. No. 9,447,301, entitled “MetalNanowire Inks for the Formation of Transparent Conductive Films withFused Networks”, which is a continuation of U.S. patent application Ser.No. 14/464,332 filed on Aug. 20, 2014 to Li et al., now U.S. Pat. No.9,150,746, entitled “Metal Nanowire Inks for the Formation ofTransparent Conductive Films With Fused Networks,” which is acontinuation of U.S. patent application Ser. No. 14/448,504 filed onJul. 31, 2014 to Li et al., now U.S. Pat. No. 9,183,968, entitled “MetalNanowire Inks for the Formation of Transparent Conductive Films withFused Networks,” incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to inks comprising metal nanowires suitable forforming transparent conductive films, especially films forming fusednanostructured metal networks. The invention further relates tostructures formed with the inks and processes to make and use the inks.

BACKGROUND OF THE INVENTION

Functional films can provide important functions in a range of contexts.For example, electrically conductive films can be important for thedissipation of static electricity when static can be undesirable ordangerous. Optical films can be used to provide various functions, suchas polarization, anti-reflection, phase shifting, brightness enhancementor other functions. High quality displays can comprise one or moreoptical coatings.

Transparent conductors can be used for several optoelectronicapplications including, for example, touch-screens, liquid crystaldisplays (LCD), flat panel display, organic light emitting diode (OLED),solar cells and smart windows. Historically, indium tin oxide (ITO) hasbeen the material of choice due to its relatively high transparency athigh conductivities. There are however several shortcomings with ITO.For example, ITO is a brittle ceramic which needs to be deposited usingsputtering, a fabrication process that involves high temperatures andvacuum and therefore is relatively slow and not cost effective.Additionally, ITO is known to crack easily on flexible substrates.

SUMMARY OF THE INVENTION

In a first aspect, the invention pertains to a metal nanowire inkcomprising from about 0.001 wt % to about 4 wt % metal nanowires, fromabout 0.05 wt % to about 5 wt % hydrophilic polymer binder, and fromabout 0.0001 to about 0.5 wt % metal ions.

In another aspect, the invention pertains to a metal nanowire inkcomprising from about 0.001 wt % to about 4 wt % metal nanowires, fromabout 0.0001 to about 0.5 wt % metal ions and from about 20 wt % toabout 60 wt % liquid alcohol in an aqueous solution at a pH from about5.5 to about 7.5 pH units.

In a further aspect, the invention pertains to a transparent conductivefilm comprising fused metal nanostructured network and a polymericpolyol, in which the film comprises from about 40 wt % to about 600 wt %polymeric polyol relative to the metal weight.

In an additional aspect, the invention pertains to a method for forminga transparent conductive network, the method comprising depositing afusing metal nanowire ink onto a substrate surface and drying the metalnanowire ink to form a transparent conductive film. The metal nanowireink can comprise from about 0.001 wt % to about 4 wt % metal nanowires,from about 0.05 wt % to about 5 wt % hydrophilic polymer binder, andfrom about 0.0001 to about 0.5 wt % metal ions. The transparentconductive film formed after the drying step can comprise fused metalnanowires in the form of a fused metal nanostructured network, thetransparent conductive film having a sheet resistance no more than about250 ohms/sq.

In other aspects, the invention pertains to a transparent conductivefilm comprising a sparse metal conductive element having a sheetresistance from about 45 ohms/sq to about 250 ohms/sq and a haze no morethan about 0.8%, or a sheet resistance from about 30 ohms/sq to about 45ohms/sq and a haze from about 0.7% to about 1.2%.

Moreover, the invention can pertain to a method for sintering an asdeposited metal nanowire ink with metal ions, the method comprisingdrying a metal nanowire film at a temperature from about 60° C. to about99° C. at a relative humidity of at least about 40% for at least about 1minute. In some embodiments, the metal nanowire film is formed throughthe deposition of a metal nanowire ink comprising from about 0.001 wt %to about 4 wt % metal nanowires, from about 0.05 wt % to about 5 wt %hydrophilic polymer binder, and from about 0.0002 to about 0.5 wt %metal ions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing fused metal network along thesubstrate surface forming a conductive pattern with a single pathway.

FIG. 2 is a schematic diagram showing fused metal nanostructured filmsalong the substrate surface forming a conductive pattern with aplurality of electrically conductive pathways.

FIG. 3 is a side view of the substrate and fused films of FIG. 2 takenalong arrow 3 in which a polymer overcoat is placed over theelectrically conductive film.

FIG. 4 is a side view of an alternative embodiment of a substrate andfused films in which electrically conductive metal leads are patternedunder an overcoat.

FIG. 5 is a top view of a patterned film with metal traces and a polymerovercoat configured for incorporation into a touch screen or othersensor device.

FIG. 6 is a schematic diagram showing the process flow for placement ofconductive metal traces in contact with a patterned film and thedeposition of a polymer overcoat over the metal traces and patternedfilm.

FIG. 7 is a schematic diagram showing a capacitance based touch sensor.

FIG. 8 is a schematic diagram showing a resistance based touch sensor.

FIG. 9 is a photograph of nine different ink systems after being storedfor two weeks without mixing.

FIG. 10 is a graph of the sheet resistance of four samples dried underdifferent conditions prior to the application of an overcoat.

DETAILED DESCRIPTION OF THE INVENTION

Stable metal nanowire inks provide for the formation of transparentconductive films with excellent optical properties and low sheetresistance in which sparse metal conductive layers generally comprisefused nanostructured metal networks with a polymer binder that areformed under controlled conditions. The inks generally comprise adispersion of metal nanowires, metal ions as a fusing agent, and apolymer binder in an aqueous system. In some embodiments, the inkcomprises an alcohol, which can lend desirable properties to the inkand/or processing into a film. In some embodiments, the stable inks havea pH toward a neutral value, as described further below, which isconvenient for processing, and effective stable fusing metal nanowireinks can be formed with no added acid. The inks can be stable withrespect to avoiding settling for significant periods of time to providefor convenient processing under commercially reasonable circumstances.The resulting transparent conductive films can have low sheet resistancealong with good optical properties, e.g., a high optical transparencyand low haze, and very low values of haze have been achieved withreasonably low values of sheet resistance. The transparent conductivefilms can be patterned through printing into a desired pattern and/or byetching a film to form a desired pattern. The transparent conductivefilms can be incorporated into a range of products such as touch sensorsor photovoltaic devices.

Silver nanowire based films have entered commercial use for theformation of transparent conductive electrodes or the like. The metal inthe nanowires is inherently electrically conductive, but in structuresformed with the metal nanowires electrical resistance can arise frominsufficient contact between wires to provide for longer distanceelectrical conduction. Efforts to improve electrical conductivity formetal nanowire films can be based on improving the properties of thenanowires along with improving the contact at junctions between adjacentmetal nanowires. It has been discovered that fusing of adjacent metalnanowires to form a fused nanostructured metal network can be a flexibleand effective approach to form transparent conductive films withdesirable properties.

As noted above, sparse metal conductive layers can be effectively formedfrom metal nanowires. In embodiments of particular interest, a metalnanostructured network can be formed from fused metal nanowires withdesirable results with respect to forming transparent conductive films.Specifically, the fused metal nanostructured networks can be formed withexcellent electrical and optical qualities, which can be patternedconveniently and can be combined with a polymer binder to form aninitially stabilized electrically conductive film. The metalnanostructured networks formed from fused metal nanowires providedesirable alternative to other nanowire based transparent conductivefilm structures.

Metal nanowires can be formed from a range of metals, and metalnanowires are available commercially. While metal nanowires areinherently electrically conducting, the vast majority of resistance inthe metal nanowires based films is believed to be due to the incompletejunctions between nanowires. Depending on processing conditions andnanowire properties, the sheet resistance of a relatively transparentnanowire film, as deposited without fusing, can be very large, such asin the giga-ohm/sq range or even higher, although analogous unfusedfilms may not necessarily be large. Various approaches have beenproposed to reduce the electrical resistance of the nanowire filmswithout destroying the optical transparency. Low temperature chemicalfusing to form a metal nanostructured network has been found to be veryeffective at lowering the electrical resistance while maintaining theoptical transparency. Using fused metal nanowire films provides forsignificant stabilization of the conductive films and provides extremelydesirable performance especially with less critical reliance on theproperties of the metal nanowires.

In particular, a significant advance with respect to achievingelectrically conductive films based on metal nanowires has been thediscovery of well controllable processes to form a fused metal networkwhere adjacent sections of the metal nanowires fuse. In particular, itwas discovered in previous work that halide ions can drive the fusing ofmetal nanowires to form fused metal nanostructures. Fusing agentscomprising halide anions were introduced in various ways to successfullyachieve the fusing with a corresponding dramatic drop in the electricalresistance. Specifically, the fusing of metal nanowires with halideanions has been accomplished with vapors and/or solutions of acidhalides as well as with solutions of halide salts. Fusing of metalnanowires with halide sources is described further in published U.S.patent applications 2013/0341074 to Virkar et al., entitled “MetalNanowire Networks and Transparent Conductive Material,” and 2013/0342221to Virkar et al. (the '221 application), entitled “Metal NanostructuredNetworks and Transparent Conductive Material,” both of which areincorporated herein by reference. The '221 application describeseffective patterning based on the selective delivery of HCl vapors forforming high electrical conductivity contrast patterns that areeffectively invisible to an ordinary observer under room lighting.

Metal halides formed along the surface of metal nanowires are believedto increase the mobility/diffusivity of the metal ions that result infusing of points of contact or near contact between nanowires to formthe fused network. Evidence suggests that a metal halide shell forms onthe resulting fused nanowire network when the halide fusing agents areused. While not wanting to be limited by theory, it is believed that themetal halide coating on the metal nanowires results in mobilization ofmetal atoms/ions from the nanowires such that the mobilized ionscondense to form joints between nearby nanowires forming thenanostructured network and presumably lowering the free energy whenforming the fused network with a net movement of metal atoms within thenanostructure.

An extension of the process for forming fused metal nanowire networkswas based on reduction/oxidation (redox) reactions that can be providedto result in fused nanowires without destroying the optical propertiesof the resulting film. Without wanting to be limited by theory, againthe driving force would seem to be a lowering of free energy through themigration of metal to junctions to form a fused nanostructured network.Metal for deposition at the junctions can be effectively added as adissolved metal salt or can be dissolved from the metal nanowiresthemselves. The effective use of redox chemistry for fusing metalnanowires into a nanostructured network is described further in U.S.patent application 2014/0238833 to Virkar et al. (the '833 application),entitled “Fused Metal Nanostructured Networks, Fusing Solutions withReducing Agents and Methods for Forming Metal Networks,” incorporatedherein by reference. The '833 application also described a singlesolution approach for the formation of fused metal nanostructurednetworks, and stable inks for single solution deposition for formingfused metal nanostructured networks with excellent performance aredescribed herein.

A further approach for the fusing of the nanowires has been describedbased on providing a high pH, i.e., alkaline, fusing solution to a metalnanowire film. See, copending U.S. patent application 2015/0144380 toYang et al. (the '380 application), entitled “Transparent ConductiveCoatings Based on Metal Nanowires and Polymer Binders, SolutionProcessing Thereof, and Patterning Approaches,” incorporated herein byreference. Generally, to achieve effective fusing, the pH can be greaterthan about 9.5 pH units. It is believed that the alkaline conditionseffectively mobilize metal ions along the surface of the metalnanowires. The metal then selectively migrates to points of contact ornear contact between adjacent metal nanowires to fuse the wires. Thus,the alkaline fusing provides another alternative to the halide basedfusing or the redox based fusing.

For some applications, it is desirable to pattern the electricallyconductive portions of the film to introduce desired functionality, suchas distinct regions of a touch sensor. Of course, patterning can beperformed simply by changing the metal loading on the substrate surfaceeither by printing metal nanowires at selected locations with otherlocations being effectively barren of metal or to etch or otherwiseablate metal from selected locations to remove at least some of themetal at the etched/ablated locations. Various masking, focusedradiation, photolithographic techniques, combinations thereof or thelike can be used to support the patterning process.

In the single solution or one ink system, dissolved metal ions can beprovided as a metal source so that corresponding inks without the metalions generally are not observed to result in fusing of the metalnanowires. For the one ink systems, it has been found that keeping thepH between about 1.5 pH units and 8 pH units or narrower ranges can bedesirable to limit metal mobility in the ink and improve stability. Atthese pH values since moderate pH values do not significantly mobilizethe metal from the wires, fusing is still observed with properformulation of the inks based on the metal ions added to the inks. Insome embodiments, suitable fusing metal nanowire inks can be formed withno added acid, although some amounts of acid can still be effectivelyused to form transparent conductive films. In some processing contexts,it can be desirable also to avoid low pH inks since the acid can becorrosive to some processing equipment, so the less acidic inks can alsobe desirable from a processing perspective. While for two ink systemssummarized above with a distinct fusing solution added to an asdeposited nanowire film, binder selection has been diverse whileproviding for good fusing and corresponding desirable film properties,for the stable one ink systems, hydrophilic polymers have been found tofacilitate fusing to obtain desired low sheet resistance in theresulting films. The fusing is believed to take place during the dryingprocess when concentrations of various constituents increased as solventis removed. As described further below, in some embodiments improvedfusing can be achieved using a more gradual drying process under humidconditions.

The desirable inks to achieve effective single deposition inks that cureinto fused nanostructured metal networks comprise a desired amount ofmetal nanowires to achieve appropriate loading of metal in the resultingfilm. In appropriate solutions, the inks are stable prior to depositionof the ink and drying. The inks can comprise a reasonable amount ofpolymer binder that contributes to the formation of a stable conductingfilm for further processing. To obtain good fusing results, hydrophilicpolymers have been found to be effective, such as cellulose or chitosanbased polymers. Metal ions, as a source of metal for the fusing process,are supplied as a soluble metal salt.

As shown in the examples, low sheet resistance films can be formedwithout an alcohol solvent with appropriate other components of theinks. In particular, some suitable polymer binders can have functionalgroups that can reduce the metal ions to drive the fusing process.However, it can be desirable for the solvent for the inks to be anaqueous alcoholic solution. In some embodiments, the solutions cancomprise from about 5 wt % to about 80 wt % of alcohol relative to thetotal liquid. Generally, the bulk of the remainder of the liquid iswater, although some amounts of other organic solvents can be used. Inadditional or alternative embodiments, a blend of alcohols has beeneffectively used. In particular, a blend of isopropyl alcohol andethanol has been found to yield inks with good stability and fusingproperties. Stability is discussed further below, but generally a stableink has no settling of solids after 1 hour of settling without stirringand limited visible separation after two weeks of settling withoutstirring.

With one ink processing, fusing metal nanowire inks with improvedstability have generally been achieved without significant acidificationof the inks while still providing for good fusing of metal nanowires.Also, to reduce corrosion of processing equipment, it can be desirablefor the pH of the ink to be not highly acidic. In some embodiments, somemild acidification can be performed to improve solubilization of thepolymer binders and/or to promote the fusing process to a furtherdegree. Acidification to a pH below about 1.5 pH units and in someembodiments below about 3 pH units generally results in an ink with anundesirable acidity. It has been discovered that good fusing of themetal nanowires is achievable without strong acidification. Thus, theformation of inks for direct processing for fused nanostructured metalnetworks surprisingly can be effectively accomplished with metal ions asthe metal source without any acidification or without strongacidification of the ink. However, there may be specific applications inwhich greater acidification can be tolerated and may provide desirableconductive films.

The use of a two solution approach with a separate fusing solution hasprovided for greater flexibility in forming patterns with fused andunfused regions with comparable metal loadings. As formed unfused filmswith a polymer binder can be formed with high sheet resistance values.The use of a single solution to form the fused metal nanostructuredmetal network suggests approaches for patterning based on metal loading.In particular, approaches for patterning include, for example, printingthe inks to directly form the pattern with regions formed free of theink, and/or removing portions of the deposited inks to lower theirconductivity. As explained further below, removal of metal loading fromregions of the substrate generally can be performed before or afterdrying to complete the fusing and/or can involve a portion orsubstantially all of the metal located at the selected region.

With one ink processing, the metal nanowires fuse to form a fusednanostructured metal network. The optical properties correspond withthat of a sparse metal layer, which can manifest good opticaltransparency with a low haze. The resulting films can exhibit distinctproperties clearly consistent with the concept that adjacent metalnanowires fuse. As an initial matter, earlier work involving a singleink fusing approach were examined with electron microscopy in whichimages showed the fused connections between adjacent wires. See, forexample, the '221 application cited above. Similar micrographs have beenobtained for the present films that show fusing of the metal nanowiresinto a fused network. Furthermore, control solutions without the metalions do not exhibit the achieved very low sheet resistances observedwith the fused systems. Additional evidence is found in data in whichthe curing/drying process is carried out in humid atmosphere. Forcertain silver nanowires, the fusing process seems to proceed slower,and the fusing can be performed over an extended period of time. Thedrying can be performed in a humid atmosphere to dry the film moreslowly and provide for additional fusing of metal structure with acorresponding observed lowering of the sheet resistance. Furthermore,importantly the fused nanostructured metal networks provide a stabilityto the resulting transparent conductive films that allow for formationof structures incorporating the films that Applicant believes have notbeen achievable with traditional unfused metal nanowire films. Thus,films with fused nanostructured metal networks are potentially moresuitable for applications in devices with stretchable and bendabletransparent conductive films than the unfused nanowire films. Formedtransparent conductive films are described in provisional U.S. patentapplication 61/978,607 to Kambe et al., entitled “Formable TransparentConductive Films with Metal Nanowires,” incorporated herein byreference.

In some embodiments of particular interest, the metal nanowire inks arestable in that no settling of solids is observed after one hour withoutstirring, or possibly much longer. Of course, for use, the inks may bestirred shortly before use to ensure a desired level of uniformity andperformance. Nevertheless, the stability of the inks can providedesirable commercial advantages with respect to shelf life andprocessing approaches. Consistent with the formation of stable inks, therheology of the inks can be adjusted over reasonable ranges to providefor commercial deposition approaches.

Various printing approaches can be used to print the inks in a patternon a substrate. For example, lithography can be used to perform thepatterning. For example, photoresists, such as commercially availablecompositions, can be used to block portions of the substrate withsubsequent removal of the resist correspondingly removing metal loadingsnot associated with the exposed substrate. Alternatively, screenprinting, spray coating, gravure printing or the like can be used toselectively deposit the metal nanowire inks onto portions of thesubstrate. Alternatively or additionally, metal loading can be partiallyremoved after deposition onto the substrate, for example through etchingor the like. Partial removal of the metal loading can comprise partialor complete removal of the metal at selected locations. Suitable etchingapproaches include, for example, radiation based etching and/or chemicaletching. Radiation based etching can be performed with focused radiationor with masking to control exposure to the radiation. For example, alaser or a focused electron beam can be used for focused radiationetching. Masking to focus radiation can also be effectively used. Withrespect to chemical etching, masking can be used to control the chemicaletching. For example, lithography can be used to form a pattern todirect chemical etching.

For incorporation into a product, protective layers are generally placedover the transparent conductive film and a stack structure is formed tohave a structure that can then be incorporated into a product. While arange of structures can be formed to adapt to a particular productspecifications, a general structure can comprise one or more layers as asubstrate, the electrically conductive film with or without patterningon the substrate and one or more top coats. A thicker laminate top coatcan be effectively used to protect the electrically conductive film.

Electrically conductive films formed with the single ink fusing processcan achieve desirable low levels of sheet resistance, for example, nomore than about 75 ohms/sq. Of course, the sheet resistance can beadjusted with various parameters, such as metal loading, and very lowlevels of sheet resistance may not be specified for lower costcomponents. Simultaneously with the low sheet resistance, the films canhave very good optical transparency and low haze. Thus, the resultingstructures are well suited for a range of applications for transparentconductive electrodes.

Ink Compositions and Properties

A single ink formulation provides for depositing a desired loading ofmetal as a film on the substrate surface and simultaneously providingconstituents in the ink that induce the fusing process as the ink isdried under appropriate conditions. These inks can be referred toconveniently as fusing metal nanowire inks with the understanding thatthe fusing generally does not take place until drying. The inksgenerally comprise an aqueous solvent, which can further comprise analcohol and/or other organic solvent in some embodiments. The inks canfurther comprise dissolved metal salts as a metal source for the fusingprocess. Without wanting to be limited by theory, it is believed thatcomponents of the ink, e.g., alcohol, reduce the metal ions fromsolution to drive the fusing process. Previous experience with thefusing process in these systems suggests that the metal preferentiallydeposits at the junctions between adjacent metal nanowires. A polymerbinder can be provided to stabilize the film and to influence inkproperties. The particular formulation of the ink can be adjusted toselect ink properties suitable for a particular deposition approach andwith specific coating properties desired. It has been found that withthe one ink system to form fused metal nanostructured networks,desirable values of sheet resistance can be achieved with hydrophilicpolymers, such as cellulose based polymers or chitosan based polymers.As described further below, drying conditions can be selected toeffectively perform the fusing process.

Thus, for improved embodiments described herein, the fusing metalnanowire inks generally can comprise metal nanowires, an aqueoussolvent, dissolved metal salts, an optional hydrophilic polymer binder,and optionally other additives. The concentration of metal nanowiresinfluences both the fluid properties of the inks and the loading ofmetal deposited onto the substrate. The metal nanowire ink generally cancomprise from about 0.001 to about 4 weight percent metal nanowires, infurther embodiments from about 0.005 to about 2 weight percent metalnanowires and in additional embodiments from about 0.01 to about 1weight percent metal nanowires. A person of ordinary skill in the artwill recognize that additional ranges of metal nanowire concentrationswithin the explicit ranges above are contemplated and are within thepresent disclosure.

In general, the nanowires can be formed from a range of metals, such assilver, gold, indium, tin, iron, cobalt, platinum, palladium, nickel,cobalt, titanium, copper and alloys thereof, which can be desirable dueto high electrical conductivity. Commercial metal nanowires areavailable from Sigma-Aldrich (Missouri, USA), Cangzhou Nano-ChannelMaterial Co., Ltd. (China), Blue Nano (North Carolina, USA), EMFUTUR(Spain), Seashell Technologies (California, USA), Aiden (Korea),Nanocomposix (USA), K&B (Korea), ACS Materials (China), KeChuangAdvanced Materials (China), and Nanotrons (USA). Alternatively, silvernanowires can also be synthesized using a variety of known synthesisroutes or variations thereof. Silver in particular provides excellentelectrical conductivity, and commercial silver nanowires are available.To have good transparency and low haze, it is desirable for thenanowires to have a range of small diameters. In particular, it isdesirable for the metal nanowires to have an average diameter of no morethan about 250 nm, in further embodiments no more than about 150 nm, andin other embodiments from about 10 nm to about 120 nm. With respect toaverage length, nanowires with a longer length are expected to providebetter electrical conductivity within a network. In general, the metalnanowires can have an average length of at least a micron, in furtherembodiments, at least 2.5 microns and in other embodiments from about 5microns to about 100 microns, although improved synthesis techniquesdeveloped in the future may make longer nanowires possible. An aspectratio can be specified as the ratio of the average length divided by theaverage diameter, and in some embodiments, the nanowires can have anaspect ratio of at least about 25, in further embodiments from about 50to about 10,000 and in additional embodiments from about 100 to about2000. A person of ordinary skill in the art will recognize thatadditional ranges of nanowire dimensions within the explicit rangesabove are contemplated and are within the present disclosure.

The solvent for the inks generally comprises an aqueous solvent thatoptionally also comprises an alcohol. An alcohol can provide reducingcapability to drive nanowire fusing and can provide desirable inkcharacteristics, such as improved coating qualities. For embodimentscomprising an alcohol, the solvent can comprise water and from about 5weight percent to about 80 wt % alcohol, in further embodiments fromabout 10 wt % to about 70 wt %, in additional embodiments from about 15wt % to about 65 wt % and in other embodiments from about 20 wt % toabout 60 wt % alcohol. Suitable alcohol generally are soluble in ormiscible with water over appropriate concentration ranges, and include,for example, short chain alcohols, such as, methanol, ethanol, isopropylalcohol, isobutyl alcohol, tertiary butyl alcohol, other alcohols withlinear or branched chains with up to 7 carbon atoms, ethylene glycol,propylene glycol, diacetone alcohol, ethyl lactate, methoxy ethanol,methoxy propanol, other glycol ethers, such as alkyl cellosolves andalkyl carbitols, or the like or blends thereof. Solvents comprising ablend of isopropyl alcohol and ethanol in an aqueous solvent aredescribed in the examples below. In some embodiments, the solvent canoptionally comprise minor amounts of other soluble organic liquidsincluding, for example, ketones, esters, ethers, such as glycol ethers,aromatic compounds, alkanes, and the like and mixtures thereof, such asmethyl ethyl ketone, glycol ethers, methyl isobutyl ketone, toluene,hexane, ethyl acetate, butyl acetate, PGMEA(2-methoxy-1-methylethylacetate), or mixtures thereof. If an optionalorganic solvent is present, the nanowire ink generally comprises no morethan about 10 weight percent non-alcohol organic solvent, in furtherembodiments from about 0.5 wt % to about 8 wt % and in additionalembodiments from about 1 wt % to about 6 wt % non-alcohol organicsolvent. A person of ordinary skill in the art will realize thatadditional ranges of solvent concentrations within the explicit rangesabove are contemplated and are within the present disclosure.

Metal ions provide the source of metal for fusing the metal nanowires,and the inks comprise an appropriate concentration of metal ions. Themetal ions are supplied as a dissolved salt within the solvent.Generally, the metal nanowire ink comprises from about 0.0001 wt % toabout 0.5 wt % metal ions, in further embodiments from about 0.00025 wt% to about 0.075 wt %, in other embodiments from about 0.0003 wt % toabout 0.06 wt %, in additional embodiments from about 0.0005 wt % toabout 0.05 wt % metal ions and in some embodiments from about 0.00075 wt% to about 0.025 wt %. A person of ordinary skill in the art willrecognize that additional ranges within the explicit ranges of metal ionconcentrations above are contemplated and are within the presentdisclosure. The metal salts also comprise a counter ion, which isgenerally believed to be inert in the film formation process, althoughthe selection of the particular salt should provide for complete andrapid solubility in the solvent of the ink. Generally, suitable anionsinclude, for example, nitrates, sulfates, perchlorates, acetates,fluorides, chlorides, bromides, iodides, and the like. The specificanion selection may influence somewhat the metal ion activity due tosome complexation with the ion in solution, and empirical adjustment canbe made based on the teachings herein. The metal ions generallycorrespond with the metal element of the nanowires, so with silvernanowires, silver salts are generally used to supply metal ions forfusing the nanowires. However, it is possible to use other metal ions orcombinations thereof if the metal element corresponding with the metalions corresponds with a metal element with an oxidation potentialapproximately comparable to or greater than, i.e., more difficult tooxidize, the metal of the nanowires. So for silver nanowire, gold ions,platinum ions, palladium ions, zinc ions, nickel ions or the like can beused in addition to or as an alternative to silver ions, and appropriateions for other nanowires can be similarly chosen based on the teachingsherein.

The pH of the fusing metal nanowire ink may or may not be adjusted withthe addition of an acid. It has been found in some embodiments thatstable metal nanowire inks can be formed without the addition of anacid, which results in an ink with a relatively neutral pH, e.g., fromabout 5.5 pH units to about 8 pH units. If no acid is added, the pH isinfluenced by the purity of the component solvents, dissolved CO₂,properties of the additives, e.g., polymer binders, and the like. It maybe desirable to add acid to facilitate dissolving of a polymer binder,to influence the properties of the transparent conductive film, toinfluence other properties of the inks or other reasons. From aprocessing perspective, an ink that is not too acidic can be desirableto reduce corrosion of process equipment. Thus, in some embodiments, itis desirable to have an ink pH from about 3 pH units to about 8 pHunits, in further embodiments from about 3.4 pH units to about 7.6 pHunits and in additional embodiments, from about 3.8 pH units to about7.3 pH units. In alternative embodiments, a more acidic ink can be used,but at high enough acidity generally the stability of the ink may becomedifficult to maintain. For more acidic stable inks, the pH generally ismaintained at values of no less than about 1.5 pH units, in furtherembodiments no less than about 1.75 and in additional embodiments fromabout 2 pH units to about 3 pH units. A person of ordinary skill in theart will realize that additional ranges of pH are contemplated and arewithin the present disclosure. In general, any reasonable acid can beused to adjust the pH, for example strong acids, such as nitric acid,sulfuric acid, perchlorate acid, hydrochloric acid, sulfonic acid, andthe like, or for higher pH values with a weak acid, such as acetic acid,citric acid, other carboxylic acids or the like. The specific acids andpH values are generally selected to avoid damage to polymer binders andother ink components.

The inks can optionally comprise a hydrophilic polymer component whichis dissolved in the solvent. The inks generally comprise from about 0.01wt % to about 5 wt % hydrophilic polymer, in additional embodiments fromabout 0.02 wt % to about 4 wt % and in other embodiments from about 0.05wt % to about 2 wt % hydrophilic polymer. A person of ordinary skill inthe art will recognize that additional ranges of hydrophilic polymerconcentrations within the explicit ranges above are contemplated and arewithin the present disclosure. As used herein, the term polymer is usedto refer to molecules with an average molecular weight of at least about1000 g/mole, and polymers of particular interest in neat form aresolids, although in some embodiments crosslinking can be introducedfollowing deposition to alter the properties of the polymer. Somepolymers can be difficult to evaluate with respect to molecular weight,and cellulose based polymers can be in this class. However, polymersthat are difficult to evaluate with respect to molecular weight aregenerally understood to be at least of moderate molecular weight andwould be recognized to have a molecular weight of greater than 500g/mole even if a more specific value is difficult to attribute to thecomposition due to the macromolecular nature of the composition.Hydrophilic polymers generally comprise polar functional groups, such ashydroxyl groups, amide groups, amine groups, acid groups and the like,and suitable polymers with multiple hydroxyl groups, which can bereferred to as polymeric polyols. Polysaccharides are a type ofpolymeric polyols that can have desirable properties for ink formation.Polysaccharides are sugar polymers or derivatives thereof with a largenumber of hydroxyl groups. Polysaccharides include, for example,cellulose based polymers and chitosan based polymers, and desirable inksbased on these binders are described in the Examples below. Cellulosebased polymers include cellulose esters and cellulose ethers that areformed by partial digestion of natural cellulose and reaction of afraction of the hydroxyl groups in the cellulose. Particular cellulosebased polymers include, for example, cellulose acetate, cellulosepropionate, ethylcellulose, methyl cellulose, hydroxyethylcellulose,hydroxypropylmethyl cellulose, hydroxyethylmethyl cellulose, and thelike. In general, commercial cellulose based polymers are notcharacterized by molecular weight, but these polymers can be assumed tohave average molecular weights in the range of polymers as specifiedherein. Similarly, chitosan is a polysaccharide produced from thereaction of chitin, a natural product found in the shells of crustaceansand in fungi. Chitosan can be characterized by the degree ofdeacetylation of the native chitin and by the molecular weight, whichgenerally ranges from about 3500 to about 220,000 g/mole. Chitosan issoluble in dilute aqueous acid solutions, and for example weakcarboxylic acids can be used to incorporate it into the nanowire inks.

While it has been discovered that the presence of a hydrophilic polymerin the ink is significant if a binder is used in the ink, additionalpolymer binders can be included effectively along with the hydrophilicpolymer. Suitable binders include polymers that have been developed forcoating applications. Crosslinkable scratch resistant protectivecoatings, which can be referred to as hard coat polymers or resins, e.g.radiation curable coatings, are commercially available, for example ascurable, e.g., crosslinkable, materials for a range of applications,that can be selected for dissolving in aqueous or non-aqueous solvents.Suitable classes of radiation curable polymers include, for example,polyurethanes, acrylic resins, acrylic copolymers, polyethers,polyesters, epoxy containing polymers, and mixtures thereof. As usedherein, polymeric polyols are not considered hardcoat polymers. Examplesof commercial polymer binders include, for example, NEOCRYL® brandacrylic resin (DMS NeoResins), JONCRYL® brand acrylic copolymers (BASFResins), ELVACITE® brand acrylic resin (Lucite International), SANCURE®brand urethanes (Lubrizol Advanced Materials), BAYHYDROL™ brandpolyurethane dispersions (Bayer Material Science), UCECOAT® brandpolyurethane dispersions (Cytec Industries, Inc.), MONWITOL® brandpolyvinyl butyral (Kuraray America, Inc.), polyvinyl acetates, mixturesthereof, and the like. The polymer binders can be self-crosslinking uponexposure to radiation, and/or they can be crosslinked withphotoinitiator or other crosslinking agent. In some embodiments,photocrosslinkers may form radicals upon exposure to radiation, and theradicals then induce crosslinking reactions based on radicalpolymerization mechanisms. Suitable photoinitiators include, forexample, commercially available products, such as IRGACURE® brand(BASF), GENOCURE™ brand (Rahn USA Corp.), and DOUBLECURE® brand (DoubleBond Chemical Ind., Co, Ltd.), combinations thereof or the like.

If a UV curable resin binder is used along with the hydrophilic binder,e.g. polymeric polyol, the ink generally comprises from about 0.01 wt %to about 2.5 wt % curable binder, in further embodiments from about0.025 wt % to about 2 wt % and in additional embodiments from about 0.05wt % to about 1.5 wt % curable binder. To facilitate the crosslinking ofthe binder, the metal nanowire ink can comprise from about 0.0005 wt %to about 1 wt % of a crosslinking agent, e.g., a photoinitiator, infurther embodiments from about 0.002 wt % to about 0.5 wt % and inadditional embodiments from about 0.005 to about 0.25 wt %. A person ofordinary skill in the art will recognize that additional ranges ofcurable binder and crosslinking agent within the explicit ranges aboveare contemplated and are within the present disclosure. Applicant hasfound in some embodiments at least that the combination of a hydrophilicbinder and a curable resin, e.g., hardcoat binder, can provideadvantageous properties to the transparent conductive films. Inparticular, the hydrophilic polymer facilitates the fusing process inthe one ink format such that desirably low sheet resistance values canbe achieved while the curable resin is believed to provide protection tothe film from environmental degradation following incorporation into aproduct.

The nanowire ink can optionally comprise a rheology modifying agent orcombinations thereof. In some embodiments, the ink can comprise awetting agent or surfactant to lower the surface tension, and a wettingagent can be useful to improve coating properties. The wetting agentgenerally is soluble in the solvent. In some embodiments, the nanowireink can comprise from about 0.01 weight percent to about 1 weightpercent wetting agent, in further embodiments from about 0.02 to about0.75 weight percent and in other embodiments from about 0.03 to about0.6 weight percent wetting agent. A thickener can be used optionally asa rheology modifying agent to stabilize the dispersion and reduce oreliminate settling. In some embodiments, the nanowire ink can compriseoptionally from about 0.05 to about 5 weight percent thickener, infurther embodiments from about 0.075 to about 4 weight percent and inother embodiments from about 0.1 to about 3 weight percent thickener. Aperson of ordinary skill in the art will recognize that additionalranges of wetting agent and thickening agent concentrations within theexplicit ranges above are contemplated and are within the presentdisclosure.

Wetting agents can be used to improve the coatability of the metalnanowire inks as well as the quality of the metal nanowire dispersion.In particular, the wetting agents can lower the surface energy of theink so that the ink spreads well onto a surface following coating.Wetting agents can be surfactants and/or dispersants. Surfactants are aclass of materials that function to lower surface energy, andsurfactants can improve solubility of materials. Surfactants generallyhave a hydrophilic portion of the molecule and a hydrophobic portion ofthe molecule that contributes to its properties. A wide range ofsurfactants, such as nonionic surfactants, cationic surfactant, anionicsurfactants, zwitterionic surfactants, are commercially available. Insome embodiments, if properties associated with surfactants are not anissue, non-surfactant wetting agents, e.g., dispersants, are also knownin the art and can be effective to improve the wetting ability of theinks. Suitable commercial wetting agents include, for example, COATOSIL™brand epoxy functionalized silane oligomers (Momentum PerformanceMaterials), SILWET™ brand organosilicone surfactant (MomentumPerformance Materials), THETAWET™ brand short chain non-ionicflurosurfactants (ICT Industries, Inc.), ZETASPERSE® brand polymericdispersants (Air Products Inc.), SOLSPERSE® brand polymeric dispersants(Lubrizol), XOANONS WE-D545 surfactant (Anhui Xoanons Chemical Co.,Ltd), EFKA™ PU 4009 polymeric dispersant (BASF), MASURF FP-815 CP,MASURF FS-910 (Mason Chemicals), NOVEC™ FC-4430 fluorinated surfactant(3M), mixtures thereof, and the like.

Thickeners can be used to improve the stability of the dispersion byreducing or eliminating settling of the solids from the metal nanowireinks. Thickeners may or may not significantly change the viscosity orother fluid properties of the ink. Suitable thickeners are commerciallyavailable and include, for example, CRAYVALLAC™ brand of modified ureasuch as LA-100 (Cray Valley Acrylics, USA), polyacrylamide, THIXOL™ 53 Lbrand acrylic thickener, COAPUR™ 2025, COAPUR™ 830 W, COAPUR™ 6050,COAPUR™ XS71 (Coatex, Inc.), BYK® brand of modified urea (BYKAdditives), Acrysol DR 73, Acrysol RM-995, Acrysol RM-8W (Dow CoatingMaterials), Aquaflow NHS-300, Aquaflow XLS-530 hydrophobically modifiedpolyether thickeners (Ashland Inc.), Borchi Gel L 75 N, Borchi Gel PW25(OMG Borchers), and the like.

Additional additives can be added to the metal nanowire ink, generallyeach in an amount of no more than about 5 weight percent, in furtherembodiments no more than about 2 weight percent and in furtherembodiments no more than about 1 weight percent. Other additives caninclude, for example, anti-oxidants, UV stabilizers, defoamers oranti-foaming agents, anti-settling agents, viscosity modifying agents,or the like.

In general, the inks can be formed with any reasonable order ofcombining components, but in some embodiments it can be convenient tostart with well dispersed metal nanowires. The metal nanowires aregenerally dispersed in water, alcohol, or blends thereof. Suitablemixing approaches can be used to blend the inks with the addition ofcomponents.

The well blended inks can be stable with respect to settling withoutcontinued stirring. For example, a stable ink can exhibit no visiblesettling after one hour without any stirring. Visible settling can beevaluated as solids on the bottom of the vessel and/or as a visibleinhomogeneity from the top to the bottom of the vessel with the ink. Insome embodiments, the fusing metal nanowire inks can exhibit no settlingout of solids for at least a day, in further embodiments for at leastthree days, and in additional embodiments at least a week, although inksmay not exhibit settling of solids from the dispersion for significantlylonger periods. In some embodiments, the fusing metal nanowire inks canexhibit no visible inhomogeneity without stirring after at least 4hours, in additional embodiments for at least a day and in furtherembodiments for at least 4 days, although inks may be stable to visibleinhomogeneity for significantly longer periods of time. A person ofordinary skill in the art will recognize that additional ranges ofsettling stability periods within the explicit ranges above arecontemplated and are within the present disclosure. Of course in acommercial setting, the inks will be stirred shortly before use toensure very well mixed solution for deposition, and some settling shouldnot disturb a good well mixed and well characterized ink for deposition.Nevertheless, stable inks provide improved shelf lives for storage ofthe completed inks before an undesirable degree of settling occurs anddecrease the attention to mixing to maintain appropriate reproducibledeposition of the inks during use of the inks. Thus, the fusing metalnanowire inks are well suited for commercial application to formtransparent conductive films.

Processing of Inks and Structures Incorporating the TransparentConductive Films

In embodiments of particular interest, a process is used in which asparse nanowire coating is initially formed with the fusing metalnanowire inks and subsequent processing provides for the fusing of themetal nanowires into a metal nanostructured network, which iselectrically conducting. The fusing process is believed to generallytake place during the drying of the film. Fused nanostructured metalfilms are generally formed on a selected substrate surface after drying.In general, the dried films have good optical properties, including, forexample, transparency and low haze. Processing can be adapted forpatterning of the film as described further below. One or more polymerovercoats can be applied over the conductive film, whether or notpatterned, to provide a protective cover and the polymer can be selectedto maintain optical transparency.

In general, suitable substrates can be selected as desired based on theparticular application. Substrate surfaces can comprise sheets of, forexample, polymers, glass, inorganic semiconductor materials, inorganicdielectric materials, polymer glass laminates, composites thereof, orthe like. Suitable polymers include, for example, polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polyacrylate,poly(methyl methacrylate), polyolefin, polyvinyl chloride,fluoropolymer, polyamide, polyimide, polysulfone, polysiloxane,polyetheretherketone, polynorbornene, polyester, polystyrene,polyurethane, polyvinyl alcohol, polyvinyl acetate,acrylonitrile-butadiene-styrene copolymer, polycarbonate, a copolymerthereof or blend thereof or the like. Furthermore, the material can havea polymer overcoat placed on the fused metal nanowire network, and theovercoat polymers can comprise the polymers listed for the substratesabove and/or the curable resins, e.g., UV curable hardcoat polymers,described above for inclusion in the inks. Moreover, other layers can beadded on top or in between the conductive film and substrate to reducereflective losses and improve the overall transmission of the stack.

For the deposition of the fusing metal nanowire ink, any reasonabledeposition approach can be used, such as dip coating, spray coating,knife edge coating, bar coating, Meyer-rod coating, slot-die, gravureprinting, spin coating or the like. The ink can have properties, such asviscosity, adjusted appropriately with additives for the desireddeposition approach. Similarly, the deposition approach directs theamount of liquid deposited, and the concentration of the ink can beadjusted to provide the desired loading of metal nanowires on thesurface.

After forming the coating with the dispersion, the nanowire network canbe dried to remove the liquid. The fusing is believed to take placeduring the drying of the liquid. The films can be dried, for example,with a heat gun, an oven, a thermal lamp or the like, although the filmsthat can be air dried can be desired in some embodiments. In general,the fusing is believed to be a low temperature process, and any heatapplication to facilitate drying is incidental to the fusing. In someembodiments, the films can be heated to temperatures from about 50° C.to about 150° C. during drying, in further embodiments from about 60° C.to about 145° C. and in additional embodiments from about 65° C. toabout 135° C. The heating to drive the drying can be performed for atleast about 30 seconds, in further embodiments from about 45 seconds toabout 2 hours and in other embodiments from about 1 minute to about 45minutes. In some embodiments, improved electrical conductivitypresumably associated with increased fusing within the network has beenobtained with drying with an addition of humidity during the dryingprocess. For example, the relative humidity can be set to about 15% toabout 75%, in further embodiments form about 20% to about 70% and inadditional embodiments from about 25% to about 65%. The correspondingtemperature when drying with added humidity can be, for example, fromabout 50° C. to about 99° C., in further embodiments from about 60° C.to about 95° C. and in other embodiments from about 65° C. to about 90°C. The humidity naturally slows the drying time relative to equivalentsystems without the humidity, although the humidity seems to improve andpossibly speeds the fusing process. A person of ordinary skill in theart will recognize that additional ranges of temperature, drying timesand humidity within the explicit ranges above are contemplated and arewithin the present disclosure. The improvement of the fusing through thedrying under more humid conditions at lower temperatures is consistentwith a chemically driven fusing process. After drying to induce thefusing process, the films can be washed one or more times, for example,with an alcohol or other solvent or solvent blend, such as ethanol orisopropyl alcohol, to removed excess solids to lower haze.

Following fusing of the metal nanowires into a network, the individualnanowires generally are no longer present, although the physicalproperties of the nanowires used to form the network can be reflected inthe properties of the fused metal nanostructured network. The metalfusing is believed to contribute to the enhanced electrical conductivityobserved and to the good optical properties achievable at low levels ofelectrical resistance. The fusing is believed to take place at points ofnear contact of adjacent nanowires during processing. Thus, fusing caninvolve end-to-end fusing, side wall to side wall fusing and end to sidewall fusing. The degree of fusing may relate to the processingconditions, as noted above in the context of the humidity. Adjustment ofprocessing conditions can be used to achieve good fusing withoutdegradation of the fused nanowire network, such that desirable filmproperties can be achieved.

The amount of nanowires delivered onto the substrate can involve abalance of factors to achieve desired amounts of transparency andelectrical conductivity. While thickness of the nanowire network can inprinciple be evaluated using scanning electron microscopy, the networkcan be relatively sparse to provide for optical transparency, which cancomplicate the measurement. In general, the fused metal nanowire networkwould have an average thickness of no more than about 5 microns, infurther embodiments no more than about 2 microns and in otherembodiments from about 10 nm to about 500 nm. However, the fusednanowire networks are generally relatively open structures withsignificant surface texture on a submicron scale, and only indirectmethods can generally be used to estimate the thickness. The loadinglevels of the nanowires can provide a useful parameter of the networkthat can be readily evaluated, and the loading value provides analternative parameter related to thickness. Thus, as used herein,loading levels of nanowires onto the substrate is generally presented asmilligrams of nanowires for a square meter of substrate. In general, thenanowire networks can have a loading from about 0.1 milligrams (mg)/m²to about 300 mg/m², in further embodiments from about 0.5 mg/m² to about200 mg/m², and in other embodiments from about 1 mg/m² to about 150mg/m². A person of ordinary skill in the art will recognize thatadditional ranges of thickness and loading within the explicit rangesabove are contemplated and are within the present disclosure.

A polymer overcoat(s) or layer(s) can be desirable to place over themetal layer, which may or may not be patterned. In general, the polymerhardcoat binders described in the previous section can be adapted foruse as polymer overcoats, although additional polymers can be used.Also, with respect to processing, the polymer overcoats can be appliedusing solutions coating techniques, or other processing approaches suchas extrusion, lamination, calendering, melt coating techniques or thelike. If there is a plurality of polymer overcoats, they may or may notbe applied using similar approaches. For solution processed overcoats,the various coating approaches described above, can be equally appliedto these layers. However, the solution processing of a polymer overcoatcan be directed to solvents that are not necessarily compatible withforming good dispersions of metal nanowires.

In general, the polymer overcoats can have average thicknesses fromabout 50 nanometers (nm) to about 25 microns, in further embodiments,from about 75 nm to about 15 microns and in additional embodiments fromabout 100 nm to about 10 microns. A person of ordinary skill in the artwill recognize that additional ranges of overcoat thicknesses within theexplicit ranges above are contemplated and are within the presentdisclosure. In some embodiments, it may be possible to select anovercoat by choice of the refractive index and thickness such that afterapplication of the overcoat the pattern of conductive and insulatingareas is less visible. Overcoats may contain conductive particles, whichcan have average particle diameters in the range from about 3 nm-20microns. The particles, i.e. conductive elements, can range from0.0001-1.0 wt % of the coating solution which generally has betweenabout 0.1-80% by weight solids. These particles can be composed ofmetals or metal coatings, metal oxides, conductive organic materials,and conductive allotropes of carbon (carbon nanotubes, fullerenes,graphene, carbon fibers, carbon black or the like) and mixtures ofaforementioned materials. While the overcoats should not achieve a highlevel of electrical conductivity, these conductive particles can allowfor thicker overcoats to be deposited and still allow for electricalconductivity to trace electrodes. Furthermore, the overcoat layer can bedeposited on the conductive or patterned film after the trace electrodesare deposited. This allows for a thicker overcoat to be used withcorresponding stabilization advantages while still allowing forelectrical conductivity to be maintained between the transparentconductive layer and the silver (or other) bus bars.

The overcoats may or may not cover the entire substrate surface. Ingeneral, the polymers can be selected for the overcoat to have goodoptical transparency. In some embodiments, the optical properties of thefilms with the polymer overcoat are not significantly different from theoptical properties described above for the electrically conductive film.

Film Electrical and Optical Properties

The fused metal nanostructured networks can provide low electricalresistance while providing good optical properties. Thus, the films canbe useful as transparent conductive electrodes or the like. Thetransparent conductive electrodes can be suitable for a range ofapplications such as electrodes along light receiving surfaces of solarcells. For displays and in particular for touch screens, the films canbe patterned to provide electrically conductive patterns formed by thefilm. The substrate with the patterned film, generally has good opticalproperties at the respective portions of the pattern.

Electrical resistance of thin films can be expressed as a sheetresistance, which is reported in units of ohms per square (Ω/□ orohms/sq) to distinguish the values from bulk electrical resistancevalues according to parameters related to the measurement process. Sheetresistance of films is generally measured using a four point probemeasurement or an equivalent process. In the Examples below, film sheetresistances were measured using a four point probe, or by making asquare using a quick drying silver paste to define a square. The fusedmetal nanowire networks can have a sheet resistance of no more thanabout 300 ohms/sq, in further embodiments no more than about 200ohms/sq, in additional embodiments no more than about 100 ohms/sq and inother embodiments no more than about 60 ohms/sq. A person of ordinaryskill in the art will recognize that additional ranges of sheetresistance within the explicit ranges above are contemplated and arewithin the present disclosure. Depending on the particular application,commercial specifications for sheet resistances for use in a device maynot be necessarily directed to lower values of sheet resistance such aswhen additional cost may be involved, and current commercially relevantvalues may be for example, 270 ohm/sq, versus 150 ohms/sq, versus 100ohms/sq, versus 50 ohms/sq, versus 40 ohms/sq, versus 30 ohms/sq or lessas target values for different quality and/or size touch screens, andeach of these values defines a range between the specific values as endpoints of the range, such as 270 ohms/sq to 150 ohms/sq, 270 ohms/sq to100 ohms/sq, 150 ohms/sq to 100 ohms/sq and the like with 15 particularranges being defined. Thus, lower cost films may be suitable for certainapplications in exchange for modestly higher sheet resistance values. Ingeneral, sheet resistance can be reduced by increasing the loading ofnanowires, but an increased loading may not be desirable from otherperspectives, and metal loading is only one factor among many forachieving low values of sheet resistance.

For applications as transparent conductive films, it is desirable forthe fused metal nanowire networks to maintain good optical transparency.In principle, optical transparency is inversely related to the loadingwith higher loadings leading to a reduction in transparency, althoughprocessing of the network can also significantly affect thetransparency. Also, polymer binders and other additives can be selectedto maintain good optical transparency. The optical transparency can beevaluated relative to the transmitted light through the substrate. Forexample, the transparency of the conductive film described herein can bemeasured by using a UV-Visible spectrophotometer and measuring the totaltransmission through the conductive film and support substrate.Transmittance is the ratio of the transmitted light intensity (I) to theincident light intensity (I_(o)). The transmittance through the film(T_(film)) can be estimated by dividing the total transmittance (T)measured by the transmittance through the support substrate (T_(sub)).(T=I/I_(o) and T/T_(sub)=(I/I_(o))/(I_(sub)/I_(o))=I/I_(sub)=T_(film))Thus, the reported total transmissions can be corrected to remove thetransmission through the substrate to obtain transmissions of the filmalone. While it is generally desirable to have good optical transparencyacross the visible spectrum, for convenience, optical transmission canbe reported at 550 nm wavelength of light. Alternatively oradditionally, transmission can be reported as total transmittance from400 nm to 700 nm wavelength of light, and such results are reported inthe Examples below. In general, for the fused metal nanowire films, themeasurements of 550 nm transmittance and total transmittance from 400 nmto 700 nm (or just “total transmittance” for convenience) are notqualitatively different. In some embodiments, the film formed by thefused network has a total transmittance (TT %) of at least 80%, infurther embodiments at least about 85%, in additional embodiments, atleast about 90%, in other embodiments at least about 94% and in someembodiments from about 95% to about 99%. Transparency of the films on atransparent polymer substrate can be evaluated using the standard ASTMD1003 (“Standard Test Method for Haze and Luminous Transmittance ofTransparent Plastics”), incorporated herein by reference. A person orordinary skill in the art will recognize that additional ranges oftransmittance within the explicit ranges above are contemplated and arewithin the present disclosure. When adjusting the measured opticalproperties for the films in the Examples below for the substrate, thefilms have very good transmission and haze values, which are achievedalong with the low sheet resistances observed.

The fused metal networks can also have low haze along with hightransmission of visible light while having desirably low sheetresistance. Haze can be measured using a hazemeter based on ASTM D1003referenced above, and the haze contribution of the substrate can beremoved to provide haze values of the transparent conductive film. Insome embodiments, the sintered network film can have a haze value of nomore than about 1.2%, in further embodiments no more than about 1.1%, inadditional embodiments no more than about 1.0% and in other embodimentsfrom about 0.9% to about 0.2%. As described in the Examples, withappropriately selected silver nanowires very low values of haze andsheet resistance have been simultaneously achieved. The loading can beadjusted to balance the sheet resistance and the haze values with verylow haze values possible with still good sheet resistance values.Specifically, haze values of no more than 0.8%, and in furtherembodiments from about 0.4% to about 0.7%, can be achieved with valuesof sheet resistance of at least about 45 ohms/sq. Also, haze values of0.7% to about 1.2%, and in some embodiments from about 0.75% to about1.05%, can be achieved with sheet resistance values of from about 30ohms/sq to about 45 ohms/sq. All of these films maintained good opticaltransparency. A person of ordinary skill in the art will recognize thatadditional ranges of haze within the explicit ranges above arecontemplated and are within the present disclosure.

Patterning

Some devices involve a patterned transparent conductive electrode, andthe transparent conductive films described herein can be correspondinglypatterned. A particular pattern of fused conductive metal nanostructurednetwork along the substrate surface generally is guided by the desiredproduct. In other words, the electrically conductive pattern generallyintroduces functionality, such as domains for a touch screen or thelike. Of course, for some product, the entire surface can beelectrically conductive, and for these application pattern generally isnot performed. For embodiments involving patterning, the proportion ofthe surface comprising the electrically conductive fused metalnanostructured network can generally be selected based on the selecteddesign. In some embodiments, the fused network comprises from about 0.25percent to about 99 percent of the surface, in further embodiments fromabout 5 percent to about 85 percent and in additional embodiment fromabout 10 percent to about 70 percent of the substrate surface. A personof ordinary skill in the art will recognize that additional ranges ofsurface coverage within the explicit ranges above are contemplated andare within the present disclosure.

As schematic examples, a fused metal nanostructured network can form aconductive pattern along a substrate surface 100 with a singleconductive pathway 102 surrounded by electrically resistive regions 104,106, as shown in FIG. 1 or patterns along a substrate surface 120 with aplurality of electrically conductive pathways 122, 124, and 126surrounded by electrically resistive regions 128, 130, 132, 134, asshown in FIG. 2 . As shown in FIG. 2 , the fused area correspond withthree distinct electrically conductive regions corresponding withelectrically conductive pathways 122, 124, and 126. A side view of thestructure with the patterned film of FIG. 2 is shown in FIG. 3 on apolymer substrate 140 with a polymer overcoat 142. Although a singleconnected conductive region and three independently connected conductiveregions have been illustrated in FIGS. 1-3 , it is understood thatpatterns with two, four or more than 4 conductive independent conductivepathways or regions can be formed as desired. For many commercialapplications, fairly intricate patterns can be formed with a largenumber of elements. In particular, with available patterning technologyadapted for the patterning of the films described herein, very finepatterns can be formed with highly resolved features. Similarly, theshapes of the particular conductive regions can be selected as desired.

An alternative embodiment is shown in FIG. 4 with metal electrodesplaced under the overcoat in contact with the electrically conductivefused metal networks. Referring to FIG. 4 , fused metal nanostructurednetworks 150, 152 are separated by electrically resistive regions 154,156, 158. The films represented by networks 150, 152 are supported onsubstrate 160. Metal electrodes 162, 164 provide electrical connectionof conductive networks 150, 152 to appropriate circuits. Polymerovercoat 166 covers and protects conductive networks 150, 152 as well asmetal electrodes 162, 164. Since the metal electrodes 162, 164 are underthe overcoat, a thicker overcoat can be used if desired withoutadversely changing performance due to electrical insulating effects ofthe overcoat. A schematic view of the top of a thin conductive filmintegrated into a sensor design is shown in FIG. 5 . Sensor 170comprises conductive metal nanostructured film sections 172, which aredisplayed as turned squares, separated by an insulating region 174,which may or may not comprise unfused metal nanowires. Metal traces 176,178, 180, 182 each connect rows of conductive films 172. Metal traces176, 178, 180, 182 comprise connective segments 184 between adjacentconductive film sections 172 as well as conductive sections that aredirected to a connection zone 186 at an edge of the sensor where themetal traces can be connected to an electrical circuit. A polymerovercoat 190 is placed over the conductive film.

Patterning based on metal loading can involve selective deposition ofthe metal nanowire inks over selected portions of the substrate surfaceand/or selective removal of the deposited metal nanowire ornanostructured films. Patterning during deposition is described above inthe context of depositing the metal nanowire inks. If the metal nanowireink is deposited over the substrate surface, selected regions can bedeveloped to remove metal from the regions before or after fusing, aswell as before or after curing of the polymer binder. The metal can beremoved through an appropriate etching or washing or other suitableprocess. For example, laser ablation of metal nanowires is described inJapanese patent 5289859B to Nissha Printing Co. Ltd., entitled “Methodof Manufacturing Conductive Pattern-Covered Body, and Conductive PatternCovered Body,” incorporated herein by reference. An acid etching agentor other suitable wet etchant can be used. Dry etching can also beperformed. The patterning of the etching/development can be performedusing a resist composition or the like. A wide range of resists, such asphotoresists can be used for patterning and are commercially available.Photolithography using light, e.g., UV light, or electron beams can beused to form high resolution patterns, and the patterning of the metalnanowire or nanostructured films can be accomplished by etching throughwindows forming the resist. Both positive tone and negative tonephotoresists can be used. Common positive tone photoresists can be used,such as FujiFilm OCG825, TOK THMR-i-P5680 and the like, and negativetone photoresist Micro Resist Technology MR-N 415 and the like.Patterning using a resist can be performed using photolithography inwhich radiation exposure and development are performed to pattern theresist. Alternatively or additionally, a resist can be printed, such aswith screen printing or gravure printing, to pattern the resist toaccomplish the patterning processed described herein. Generally, forembodiments in which the electrically insulating region has less metalloading than the electrically conductive regions, the electricallyinsulating regions can have at least a factor of 1.5 less metal loading,in some embodiments at least a factor of 5 less metal loading, infurther embodiments at least a factor of 10 lower metal loading and inother embodiments at least a factor of 20 less metal loading relative tothe electrically conductive regions. In some embodiments, theelectrically insulating regions can be approximately devoid of metal. Aperson of ordinary skill in the art will recognize that additionalranges of decreased metal loadings within the explicit ranges above arecontemplated and are within the present disclosure.

In some embodiments, the metal nanostructured films can be used as areplacement for other materials, such as thin films of conductive metaloxides, such as indium tin oxide. For example, a roll of polymer withfused metal nanostructured films can be incorporated into a processscheme. A polymer overcoat can be placed down prior to patterning.Patterning, such as with laser etching or masking with wet or dryetching, can be used to form desired patterns of electrically conductivefilms separated by regions where at least some of the metal is removed.The polymer overcoat can be replaced or completed such as with anadditional layer or layers of overcoat. Metal traces or currentcollectors can be placed over the overcoat or penetrating through theovercoat or portion thereof. Adding some conductive diluents to thepolymer overcoat can decrease the resistance of the overcoat withoutshort circuiting the conductive pattern.

In additional or alternative embodiments, patterning can be performedprior to placement of a polymer overcoat. Referring to FIG. 6 , aprocess flow is depicted with flow arrows indicating a process flow,which generally corresponds with a temporal flow but may or may notcorrespond with physical movement. In the first view, a substrate 250 isshown with a patterned film with conductive regions 252 andnonconductive regions 254. While the figure indicates a particularsubstrate material, i.e., heat stabilized PET polymer with an additionalpolymer hardcoat layer, the process can generally be performed with anyreasonable substrate. In some embodiments, conductive regions 252comprise fused metal nanostructured networks and the nonconductiveregions 254 comprise a lower metal loading due to, for example, etchingor selective printing of the fusing metal nanowire ink. Referring to themiddle view of FIG. 6 , metal current collectors or traces 256 aredeposited in contact with conductive regions 252. While metal traces 256can be deposited and/or patterned using any reasonable process, in someembodiments, a conductive silver or copper paste can be screen printedand heated to form the metal traces. In some embodiments, silver, copperor other metallic traces can be deposited by plating, thermaldecomposition, evaporation, sputtering, or other reasonable thin filmdeposition techniques. In the last view of FIG. 6 , a polymer overcoat260 is placed over the coated substrate 250 to cover metal traces 256.

Touch Sensors

The transparent conductive films described herein can be effectivelyincorporated into touch sensors that can be adapted for touch screensused for many electronic devices. Some representative embodiments aregenerally described here, but the transparent conductive films can beadapted for other desired designs. A common feature of the touch sensorsgenerally is the presence of two transparent conductive electrodestructures in a spaced apart configuration in a natural state, i.e.,when not being touched or otherwise externally contacted. For sensorsoperating based on capacitance, a dielectric layer is generally betweenthe two electrode structures. Referring to FIG. 7 , a representativecapacitance based touch sensor 302 comprises a display component 304, anoptional bottom substrate 306, a first transparent conductive electrodestructure 307, a dielectric layer 308, such as a polymer or glass sheet,a second transparent conductive electrode structure 310, optional topcover 312, and measurement circuit 314 that measures capacitance changesassociated with touching of the sensor. Referring to FIG. 8 , arepresentative resistance based touch sensor 340 comprises a displaycomponent 342, an optional lower substrate 344, a first transparentconductive electrode structure 346, a second transparent conductiveelectrode structure 348, support structures 350, 352 that support thespaced apart configuration of the electrode structures in their naturalconfiguration, upper cover layer 354 and resistance measuring circuit356.

Display components 304, 342 can be, for example, LED based displays, LCDdisplays or other desired display components. Substrates 306, 344 andcover layers 312, 354 can be independently transparent polymer sheets orother transparent sheets. Support structures can be formed from adielectric material, and the sensor structures can comprise additionalsupports to provide a desired stable device. Measurement circuits 314,356 are known in the art.

Transparent conductive electrodes 306, 310, 346 and 348 can beeffectively formed using fused metal networks, which can be patternedappropriately to form distinct sensors, although in some embodiments thefused metal networks form some transparent electrode structures whileother transparent electrode structures in the device can comprisematerials such as indium tin oxide, aluminum doped zinc oxide or thelike. Fused metal networks can be effectively patterned as describedherein, and it can be desirable for patterned films in one or more ofthe electrode structures to form the sensors such that the plurality ofelectrodes in a transparent conductive structure can be used to provideposition information related to the touching process. The use ofpatterned transparent conductive electrodes for the formation ofpatterned touch sensors is described, for example, in U.S. Pat. No.8,031,180 to Miyamoto et al., entitled “Touch Sensor, Display With TouchSensor, and Method for Generating Position Data,” and published U.S.patent application 2012/0073947 to Sakata et al., entitled “Narrow FrameTouch Input Sheet, Manufacturing Method of Same, and Conductive SheetUsed in Narrow Frame Touch Input Sheet,” both of which are incorporatedherein by reference.

EXAMPLES

Commercial silver nanowires were used in the following examples with anaverage diameter of between 25 and 50 nm and an average length of 10-30microns. The silver nanowires (AgNWs) films were formed using thefollowing procedure. Commercially available silver nanowires (AgNWs)were obtained from the supplier in an aqueous dispersion or weredispersed in solvent to form an aqueous AgNW dispersion. The AgNWsdispersions were typically in the 0.05-1.0 wt % range. The dispersionswere then combined with one or more solutions comprising the othercomponents of the metal nanowire ink, which is optionally in an alcoholsolvent. The resulting dispersion or ink was then deposited on thesurface of a polyethylene terephthalate (PET) sheet using a hand-drawnrod approach or by blade coating. The AgNWs film was then treated withheat in an oven to cure the films as described in the specific examplesbelow.

The hydrophilic binder when used was first dissolved in water to obtaina clear solution. It is then mixed with AgNW and other components of inkwith stirring to form a homogeneous suspension, referred to as the baseink. The base ink typically contains from 0.1 wt % to 1 wt % of binderas prepared. After combining with the remaining ingredients (metal ions)in an appropriate fusing solution to obtaining the final coatingsolution, AgNW typically is present at a level between 0.1 to 1.0 wt %and the binder at about 0.01 to 1 wt %.

The fusing solutions are composed of appropriate metal salts dissolvedin appropriate solvents. The fusing solutions generally containedbetween 0.05 mg/mL (0.005 wt %) and 5.0 mg/mL (0.5 wt %) metal ions

The total transmission (TT) and haze of the AgNWs film samples weremeasured using a Haze Meter with films on a polymer substrate. To adjustthe haze measurements for the samples below, a value of substrate hazecan be subtracted from the measurements to get approximate hazemeasurements for the transparent conductive films alone. The instrumentis designed to evaluate optical properties based on ASTM D 1003 standard(“Standard Test Method for Haze and Luminous Transmittance ofTransparent Plastics”), incorporated herein by reference. The totaltransmission and haze of these films include PET substrate which hasbase total transmission and haze of ˜92.9% and 0.15%-0.40%,respectively. Sheet resistance was measured with a 4-point probe methodunless indicated otherwise. In the following examples, several differentformulations of fusing metal nanowire inks are presented along withoptical and sheet resistance measurements.

Sheet resistance was measured with a 4-point probe method, a contactlessresistance meter or using a square of silver pastes as follows. To makemeasurements prior to formation, a square of silver paste was sometimeused by painting the paste onto the surface of the samples to define asquare, or a rectangular shape, which were then annealed at roughly 120°C. for 20 minutes in order to cure and dry the silver paste. Alligatorclips were connected to the silver paste, and the leads were connectedto a commercial resistance measurement device. Electrical connectionsare made to exposed end sections of the film.

Example 1 Fusing Nanowire Inks with Hydrophilic Binder and Nitric Acid

This example tests the ability of a cellulose based polymer (CBP) to actas a binder and thickener for AgNW inks without interfering with thefusing process.

Initial AgNWs dispersions comprised deionized water and isopropylalcohol. The inks also contained a binder of CBP as described above. Inthis Example, 15 samples were prepared using the base ink. Fusingsolutions or ethanol were combined with some samples each in a ratio of3:1 AgNW ink to fusing solution or ethanol by volume. The fusingsolutions contained silver nitrate as specified above and between 15μL/mL and 80 μL/mL of HNO₃ in ethanol. The inks were then coated on aPET substrate using a Meyer rod or blade coating.

To dry the films, the films were heated in an oven in ambient atmosphereat 100° C. for 10 min. The properties of the films after heating arecompared in Table 1. Films that were formed with an ink that includedfusing solution had a reduced resistance compared to films withoutfusing solutions, which evidences the fusing of the metal nanowires inthe relevant samples. All of the samples exhibited good opticalproperties based on transparency and haze.

TABLE 1 Sample Resistance (Ω/□) % TT % Haze AgNW Ink 461 91.0 1.76 AgNWInk   >20K 91.6 1.19 AgNW Ink 2300  91.6 1.32 Ag NW Ink +  65 91.1 1.43Fusing Solution Ag NW Ink + 138 91.5 1.09 Fusing Solution Ag NW Ink +116 91.6 1.04 Fusing Solution Ag NW Ink + 131 91.5 1.08 Fusing SolutionAg NW Ink + 122 91.6 1.05 Fusing Solution Ag NW Ink +  84 91.8 0.93Fusing Solution Ag NW Ink +  76 91.6 1.11 Fusing Solution Ag NW Ink + 80 91.9 0.99 Fusing Solution Ag NW Ink +  80 91.7 0.99 Fusing SolutionAg NW ink + 693 91.4 1.33 Ethanol

Example 2 Fusing Solution Compositions

This example tests the ability of various formulations of compositionsto act as a fusing solution for AgNW inks to form desirable transparentconductive films.

Initial AgNWs dispersions comprised a solvent of deionized water and asmall amount of isopropyl alcohol. The base ink also contained a binderof CBP as described above. Fusing solutions or ethanol were combinedwith form 12 distinct samples each in a ratio of 3:1 or 4:1 AgNW ink tofusing solution or ethanol by volume, and two additional samples wereprocessed as the base ink. The fusing solutions contain between 0.05mg/mL and 5 mg/mL metal ions and between 15 μL/mL and 80 μL/mL of HNO₃in ethanol (samples 7-10) or half of the above concentration of theingredients (samples 11-14). The inks were then coated on a PETsubstrate using a Meyer rod or blade coating.

The films were then heated in an oven in ambient atmosphere at 100° C.for 10 min to dry the films. The properties of the films after heatingare compared in Table 2. Films that were formed from inks that includedfusing solution had a reduced resistance compared to films withoutfusing solutions, which indicates fusing of the nanowires in therespective films. All of the samples exhibited good optical properties,but the samples with more dilute fusing solutions exhibited slightlyhigher sheet resistance and greater haze.

TABLE 2 Ratio Fusing Solution (Ink:Fusing Resistance Sample or EthanolSolution or Ethanol) (Ω/□) % TT Haze 1 — — 975 91.4 1.53 2 — — 956 91.51.38 3 Ethanol 3:1 730 91.7 1.06 4 Ethanol 3:1 785 91.7 1.11 5 Ethanol3:1 1052 91.7 1.17 6 Ethanol 3:1 3941 91.8 1.12 7 Fusing Solution 3:1 8091.7 1.21 8 Fusing Solution 3:1 71 91.6 1.19 9 Fusing Solution 3:1 7691.6 1.21 10 Fusing Solution 3:1 71 91.6 1.27 11 Fusing Solution 4:1 12691.5 1.24 12 Fusing Solution 4:1 90 91.6 1.33 13 Fusing Solution 4:1 10691.6 1.24 14 Fusing Solution 4:1 85 91.6 1.36

Example 3 Effect of Different AgNW Samples

This example tests the suitableness of various AgNW samples to act as anAgNW source for one ink systems.

AgNWs inks were created in deionized water using AgNW from two differentcommercial sources (A and B). The inks contained CBP as the binder asdescribed above. Fusing solutions or ethanol were then combined withsome samples each in a ratio of 3:1 AgNW ink to fusing solution orethanol by volume. The fusing solutions were composed of between 0.05mg/mL and 5.0 mg/mL metal ions with between 8 μL/mL and 80 μL/mL of HNO₃in ethanol. The inks were then coated on a PET substrate using a Meyerrod or blade coating.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after heating are compared in Table3. Films formed with AgNWs from source A had higher sheet resistancewhen a fusing solution was not used and a lower sheet resistance when afusing solution was used when compared to films made with AgNWs fromsource B. However, films formed from inks with AgNWs from source B had agreater transparency and a lower haze.

TABLE 3 AgNW Fusing Solution Resistance Source or Ethanol (Ω/□) % TT %Haze A None 1903 91.5 1.46 A None 945 91.5 1.51 A Ethanol 1592 91.8 1.17A Ethanol 1847 91.7 1.23 A Fusing Solution 117 91.7 1.26 A FusingSolution 87 91.7 1.31 A Fusing Solution 82 91.8 1.20 A Fusing Solution83 91.8 1.14 B None 376 92.0 1.23 B None 342 91.8 1.31 B Ethanol 63392.2 0.91 B Ethanol 664 92.2 0.96 B Fusing Solution 115 92.1 1.07 BFusing Solution 119 92.1 0.95 B Fusing Solution 123 92.2 0.96 B FusingSolution 132 92.1 0.98

Example 4 Effect of Various Heavy Metal Ions in a One-Ink System

This example tests the effect of different heavy metal ions on theResistance, Transmittance and Haze of resulting films in a one ink AgNWsystem.

Initial AgNWs dispersions comprised a solvent of isopropyl alcohol. Thestock inks contained CBP as binder and also a wetting agent in water. Afusing solution was combined with the ink in a ratio of 1:1 AgNW ink tofusing solution by volume. The fusing solutions contained differentmetal ions in ethanol as described above. The inks were then coated on aPET substrate using a Meyer rod or blade coating.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after heating are compared in Table4. In general, films created with special metal ions like Ni(II) andAg(I) in the fusing solution had lower sheet resistances relative tocontrol sample E, while other metal ions (Co(II) and Cu(II)) did notexhibit fusing behavior.

TABLE 4 Fusing Solution Resistance Sample (metal ion) (Ω/□) % TT % HazeA Cu(II) 161 92.1 1.13 B Ni(II) 63 92.1 1.11 C Co(II) 91 92.2 1.11 DAg(I) 50 92.1 1.08 E EtOH 90 92.0 1.08

Example 5 Ink Formulation and Stability for Inks without Acid

This example tests the effect of various formulations on the stabilityof the ink and the other properties in a one ink AgNW system.

Stock AgNWs inks were created in deionized water from three differentcommercial sources (A, B, and C) to form the base ink as describedabove. Results based on AgNWs from a fourth supplier D are summarizedbelow. Three different solution compositions were formed from each ofthe three stock inks, resulting in 18 distinct samples. The firstsolution (#1) was created by mixing the stock ink with the fusingsolution immediately prior to coating. The fusing solution containsbetween 0.05 mg/mL and 5.0 mg/mL silver ions in ethanol and was added tothe #1 solution in a ratio of 1:1, stock ink to fusing solution byvolume. The second solution (#2) was created by mixing the stock inkwith a metal ion stock solution and storing until use. The metal ionstock solution contains between 50 mg/mL and 200 mg/mL metal ions indeionized water. The amount of metal ions in the blend of stock ink andstock metal ion solution was the same as in first solution (#1).Immediately prior to coating, ethanol was added to the #2 solution in aratio of 1:1, solution to ethanol by volume. The third solution (#3) wasformed by mixing the stock ink with fusion solution and storing untiluse. The fusion solution contains between 0.05 mg/mL and 5.0 mg/mL metalions in ethanol and was added to the #3 solution in a ratio of 1:1,stock ink to fusion solution by volume. The #3 solutions were stored andthen directly coated without any further mixing. The inks were coated ona PET substrate using a Meyer rod or blade coating. The stability of theinks is shown in FIG. 9 . FIG. 9 depicts each of the solutions afterbeing stored for two weeks without mixing.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after fusing are compared in Table5A. These results confirm that low sheet resistances indicating fusingof silver nanowires can effectively take place without any added acid inthe inks. Excellent optical properties based on high % TT and low hazewere also observed. The nanowires supplied by Supplier C resulted infilms with somewhat greater haze. The processing order did notsignificant alter the results suggestive of a stable ink.

TABLE 5A Untreated Cured Resistance Untreated Untreated Resistance CuredCured Sample Ink (Ω/□) % TT Haze (Ω/□) % TT Haze  1 #1-A >20K 92.0 0.8656 91.6 1.05  2 #1-A >20K 91.9 0.88 57 91.5 1.06  3 #2-A >20K 91.9 0.9058 91.4 1.11  4 #2-A >20K 91.9 0.85 56 91.4 1.08  5 #3-A 3435 91.9 0.8866 91.5 1.05  6 #3-A 1280 91.9 0.90 54 91.5 1.05  7 #1-B  214 91.8 1.0448 91.6 1.08  8 #1-B  192 91.8 1.02 48 91.6 1.05  9 #2-B 2985 91.7 0.9349 91.7 0.99 10 #2-B  443 91.8 0.96 52 91.7 1.02 11 #3-B  335 91.8 0.9852 91.7 0.99 12 #3-B  176 91.8 0.99 50 91.7 1.01 13 #1-C  396 91.8 1.1970 91.6 1.28 14 #1-C  531 91.7 1.23 67 91.6 1.30 15 #2-C  796 91.7 1.2756 91.5 1.33 16 #2-C 5331 91.7 1.27 57 91.6 1.37 17 #3-C  717 91.7 1.2158 91.5 1.42 18 #3-C 1126 91.7 1.27 57 91.5 1.38

For AgNWs from the fourth supplier D, The solution was created by mixingthe stock ink with the fusing solution immediately prior to coating. Thefusing solution contains between 0.05 mg/mL and 5.0 mg/mL metal ions inethanol and was added to the solution in a ratio of 1:1, stock ink tofusing solution by volume. Three different solutions were prepared withmixing increasing metal ion concentrations C1<C2<C3. Films were thenformed from these solutions as described above shortly after mixing thefusing solution with the nanowire stock solutions. The results arepresented in Table 5B below. The transparent conductive films formedwith these nanowires had extremely low values of haze for a particularvalue of sheet resistance. Correspondingly, at comparable values ofsheet resistance as sample formed with nanowires A or B in Table 5A,values of haze are generally lower by 0.2% or more.

TABLE 5B Untreated Cured Resistance Untreated Untreated Resistance CuredCured Sample Ink (Ω/□) % TT Haze (Ω/□) % TT Haze 1 C1-1  368 92.1 0.8055 91.9 0.83 2 C1-2  284 92.2 0.80 57 92.0 0.82 3 C2-1 1116 92.1 0.73 5991.6 0.80 4 C2-2  438 92.2 0.72 63 91.7 0.81 5 C3-1  473 92.2 0.71 6191.6 0.80 6 C3-2  324 92.2 0.73 57 91.6 0.82

Example 6 the Effect of Chitosan Binders in One Ink Formulations

This example tests the effect of chitosan binders on the resistance,transmittance, and haze in a one ink AgNW system.

Initial AgNWs dispersions comprised a solvent of deionized water and asmall amount of isopropyl alcohol. The stock AgNW inks contained between0.1 wt % and 0.3 wt % silver nanowires, between 0.01 wt % and 0.1 wt %LUVITEC® (commercially available from BASF) as a codispersant, between0.3 wt % and 0.5 wt % chitosans of different grades as binder, andbetween 0.05 wt % and 0.1 wt % of a wetting agent. The inks were mixedwith a fusing solution between 0.5 mg/mL and 5 mg/mL metal ions inethanol or with ethanol solvent in a ratio of 1:1 ink to fusing solutionor ethanol by volume. The inks were then coated on a PET substrate usinga Meyer rod or blade coating.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after fusing are compared in Table 6.All of the films made with inks containing fusing solution exhibitedexcellent properties.

TABLE 6 Fusing Solution Resistance Binder or Ethanol (Ω/□) TT % % HazeChitosan 1 Fusing Solution 57.7 91.6 1.01 Chitosan 1 Fusing Solution50.0 91.5 1.08 Chitosan 2 Fusing Solution 54.0 91.6 1.00 Chitosan 2Fusing Solution 50.3 91.6 1.03 Chitosan 3 Fusing Solution 56.3 91.6 1.01Chitosan 3 Fusing Solution 53.0 91.6 1.03 Chitosan 4 Fusing Solution64.0 91.7 0.96 Chitosan 4 Fusing Solution 58.3 91.6 0.98 Chitosan 5Fusing Solution 55.0 91.5 1.04 Chitosan 5 Fusing Solution 58.3 91.6 1.01Chitosan 6 Fusing Solution 51.7 91.6 1.02 Chitosan 6 Fusing Solution51.0 91.6 1.03 Chitosan 6 Ethanol 916 90.9 1.25

Example 7 the Effect of a Blend of Polyvinylpyrrolidone and CBP as aBinder

This example tests the effect of polyvinylpyrrolidone as a binder on theresistance, transmittance, and haze in a one ink AgNW system.

AgNWs inks were created in deionized water from two differencecommercial sources (A and B. The stock AgNW inks contains between 0.1 wt% and 0.3 wt % silver nanowires, between 0.3 wt % and 0.75 wt % CBP asbinder, between 0.01 wt % and 0.1 wt % polyvinylpyrrolidone (PVP) as adispersant/binder, and between 0.05 wt % and 0.1 wt % of a wettingagent. The inks were mixed with a fusing solution containing between 0.5mg/mL and 5 mg/mL metal ions in ethanol in a ratio of 1:1 by volume. Theinks were then coated on a PET substrate using a Meyer rod or bladecoating.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after fusing are compared in Table 7.

TABLE 7 Ink Resistance Source PVP Source (Ω/□) TT % % Haze A PVP 55K48.3 91.2 1.56 A PVP 55K 44.3 91.1 1.62 A PVP 360K 46.0 91.2 1.57 A PVP360K 46.0 91.1 1.59 A PVP 40K 44.0 91.1 1.64 A PVP 40K 43.0 91.2 1.63 APVP 10K 46.0 91.2 1.59 A PVP 10K 45.7 91.2 1.66 B PVP 55K 70.3 92.0 1.13B PVP 55K 55.7 92.0 1.15 B PVP 360K 59.0 92.0 1.11 B PVP 360K 59.7 92.01.14 B PVP 40K 75.3 91.6 1.06 B PVP 40K 68.0 91.6 1.10 B PVP 10K 73.791.5 1.10 B PVP 10K 64.3 91.6 1.12

Example 8 Effect of Moisture on Film Performance

This example tests the effect of moisture during the drying process onfilm performance.

The stock AgNW ink contains between 0.1 wt % and 0.30 wt % silvernanowires in isopropanol from source A. The inks were mixed with afusing solution containing between 0.5 mg/mL and 5 mg/mL metal ions inethanol in a ratio of 1:1 by volume. Immediately prior to coating oneadditional ink was mixed with ethanol in a ratio of 1:1, ink to ethanol.The inks were then coated on a PET substrate using a Meyer rod or bladecoating.

The films were then heated in an oven at 85° C. with a relative humidityof 60% for 20, 15, or 5 minutes, or at 130° C. in a dry oven for 5minutes to dry the films. The sheet resistance of the films after dryingunder different conditions is compared in FIG. 10 . The introduction ofhumidity allows for a reduction of both time and temperature to achievethe same level of fusing (reduction in sheet resistance) for at leastsome of the samples. The drying in the humid atmosphere provided forimproved fusing for these inks as indicated by a reduction of sheetresistance.

Example 9 Robustness of Film by Adding UV Curable Polymer to the Ink

This example tests the effect of UV curable polymer on the robustness ofthe AgNW film.

A base AgNW ink is created by mixing a AgNW ink with a AgNWconcentration of between 0.1 wt % and 0.3 wt % in isopropanol withbetween 2 wt % and 7.5 wt % of a UV curable polymer in propylene glycolmethyl ether (PGME) in a 4:1 ink to UV curable polymer ratio by volume.The inks were mixed with a fusing solution containing a concentration C1between 0.05 mg/mL and 0.5 mg/mL silver ions or C2=10×C1 in ethanol in aratio of 1:1 by volume. The inks were then coated on a PET substrateusing a Meyer rod coating at a setting of 10 or 20.

The films were then cured using a UV conveyer. The properties of thefilms are compared in Table 8. The introduction of UV curable resinscaused high haze in the coatings, but resistance to abrasion after UVcuring was obviously much improved. The films formed without the fusingsolution had very high sheet resistance values. Thus, these films withthe UV curable composition mixed into the fusing ink provides greatercontrast in electrical resistance between the films formed with thefusing solution and the films formed without the fusing solution. Thedata further show that the films with the UV curable composition mixedinto the fusing ink could exhibit remarkable thermal stability. Thebase-coatings after UV curing showed no increase in sheet resistance (R)over the original values (Ro) after a treatment of 30 min in a 150° C.oven (Samples 4 and 6).

TABLE 8 R/Ro Fusing Resistance (150° C., Sample Solution Rod (Ω/□) % TTHaze 30 min) 1 None 10 >200M 92.4 3.60 — 2 None 20 >200M 91.2 3.34 — 3C1 10 ~3,400 92.1 2.53 — 4 C1 20    60 88.7 4.65  0.87 5 C2 10   14391.2 2.16 — 6 C2 20    42 86.5 4.91 0.9

Example 10 Effect of Alcohol Free Fusing Solutions

This example tests the effect of using alcohol free fusing solutions onfilm performance.

The stock AgNW ink contained silver nanowires in isopropanol from sourceA, or in water from source B. The inks contained between 0.05 wt % and0.3 wt % AgNW. The inks were mixed with a fusing solution in water in aratio of 1:1 ink to fusing solution containing between 0.05 mg/mL and5.0 mg/mL metal ions by volume. Comparative samples were preparedsimilarly with water instead of fusing solutions. The inks were thencoated on a PET substrate using a Meyer rod or blade coating.

The films were then heated in an oven at 100° C. for 10 minutes to drythe films. The properties of the films after the heating step are shownin Table 9. The use of fusing solutions free of alcohol but with metalions leads to effective conductivity improvement in relation to therespective comparative examples, as indicated in Table 9 as“Improvement”, expressed as percent of resistance reduction.

TABLE 9 AgNW Fusing Resistance Sample Source Solution (Ω/□) Improvement% TT Haze 1 A H₂O 94 — 91.8 1.08 2 A 1 (in H₂O) 58 38% 91.7 1.11 3 A 2(4 × conc 56 40% 91.6 1.14 of 1 in H₂O) 4 B H₂O 82 — 91.8 1.20 5 B 1 (inH₂O) 60 27% 91.7 1.21 6 B 2 (4 × conc 64 21% 91.5 1.27 of 1 in H₂O)

Example 11 Effect of Polyox as Binders on AgNW Inks

This example tests the effect of Polyox binders in one ink AgNW systems.

Initial AgNWs dispersions comprised a solvent of deionized water and asmall amount of isopropyl alcohol. The AgNW inks have between 0.05 wt %and 0.25 wt % silver nanowires. The inks also contained a binder ofPolyox (polyethylene oxide) in a concentration between 0.075 wt % and0.10 wt %, a wetting agent in a concentration between 0.10 wt % and 0.15wt %. A fusing solution was added in a ratio of 3:1 AgNW ink to fusingsolution by volume. The fusing solution contained between 0.05 mg/mL and5.0 mg/mL silver ions and between 15 μL/mL and 80 μL/mL of HNO₃ inethanol. The inks were then coated on a PET substrate using a Meyer rodor blade coating.

The films were then heated in an oven at 80° C. for 10 min to dry thefilms. The properties of the films after heating are compared with filmsformed with CBP and with fusion solution or with just EtOH as control inTable 10. In general, the ink with the Polyox binder resulted in a lowresistance film when fusing solution was used indicating silver nanowirefusing. With inks having the Polyox binder, the films formed without thefusing solution had a high sheet resistance.

TABLE 10 Fusing Resistance Sample Binder Solution (Ω/□) % TT % Haze 1CBP-60SH-10k — 203 92.0 1.05 2 CBP-60SH-10k Yes 115 91.9 1.06 3 PolyoxN16 —   >20K 92 1.30 4 Polyox N16 Yes 185 91.9 1.36 5 Polyox N60 —  >20K 92 1.19 6 Polyox N60 Yes 164 91.9 1.42 7 Polyox 310 —   >20K 92.20.89 8 Polyox 310 Yes 1720  92.2 0.88

Example 12 Effect of Sodium Metal Ions in a One-Ink System

This example tests the effect of sodium metal ions on the Transmittanceand Haze in a one ink AgNW system.

Initial AgNWs dispersions comprised a solvent of deionized water and asmall amount of isopropyl alcohol. The inks contained CBP as the binderas described above. Fusing solutions containing between 0.05 mg/mL and5.0 mg/mL metal ions or ethanol were then combined with some sampleseach in a ratio of 3:1 AgNW ink to fusing solution or ethanol by volume.The fusing solutions were composed of metal ions (Na or Ag) with HNO₃ inethanol. The fusing solutions were added in two concentrations, theinitial concentration of <1 wt %, or a concentration ten times (10×) theinitial concentration. The inks were then coated on a PET substrateusing a Meyer rod or blade coating.

The films were then heated in an oven at 100° C. for 10 min to dry thefilms. The properties of the films after fusing are compared in Table11. The results suggest that the films formed with the sodium ions inthe fusing solution did not exhibit fusing, while the films formed withthe silver ions did undergo fusing.

TABLE 11 Fusing Solution Resistance (Ω/□) % TT % Haze 1 Na (1x) 107 92.00.98 2 Na (10x) 1078 92.3 1.81 3 Ag (1x) 55 91.9 1.01 4 Ag (10x) 46 91.61.28

The embodiments above are intended to be illustrative and not limiting.Additional embodiments are within the claims. In addition, although thepresent invention has been described with reference to particularembodiments, those skilled in the art will recognize that changes can bemade in form and detail without departing from the spirit and scope ofthe invention. Any incorporation by reference of documents above islimited such that no subject matter is incorporated that is contrary tothe explicit disclosure herein.

What is claimed is:
 1. A metal nanowire ink comprising an aqueous solvent, from about 0.001 wt % to about 4 wt % metal nanowires, from about 0.00025 to about 0.5 wt % metal ions, and fluoride anions.
 2. The metal nanowire ink of claim 1 wherein the aqueous solvent comprises from about 5 wt % to about 60 wt % liquid alcohol.
 3. The metal nanowire ink of claim 2 wherein the liquid alcohol is ethanol.
 4. The metal nanowire ink of claim 1 wherein a pH of the ink is at least about 3 pH units.
 5. The metal nanowire ink of claim 1 wherein a pH of the ink is from about 5.5 to about 7.5 pH units.
 6. The metal nanowire ink of claim 1 further comprising from about 0.05 wt % to about 5 wt % solid hydrophilic polymer binder soluble in the aqueous solvent.
 7. The metal nanowire ink of claim 1 wherein the hydrophilic polymer binder comprises a polymeric polyol.
 8. The metal nanowire ink of claim 1 wherein the hydrophilic polymer binder comprises a polysaccharide.
 9. The metal nanowire ink of claim 1 wherein the hydrophilic polymer binder comprises a cellulose based polymer or chitosan based polymer.
 10. The metal nanowire ink of claim 1 wherein the metal nanowires comprise silver nanowires and the metal ions comprise silver ions.
 11. The metal nanowire ink of claim 10 comprising from about 0.01 wt % to about 2 wt % of the silver nanowires, and from about 0.001 to about 0.2 wt % of the silver ions, and the metal nanowire ink further comprising from about 0.1 wt % to about 2 wt % of a polysaccharide polymer binder.
 12. The metal nanowire ink of claim 1 wherein the metal ions and the fluoride anions are provided as silver fluoride.
 13. The metal nanowire ink of claim 1 comprising about 0.0001 to about 0.1 wt % fluoride ions.
 14. The metal nanowire ink of claim 1 wherein no visible settling can be observed after one hour of sitting without stirring.
 15. The metal nanowire ink of claim 1 further comprising from about 0.01 wt % to about 2.5 wt % curable binder. 